One of the future challenges for the aviation industry is to develop suitable strategies for the maintenance and extension of service life of the existing aircraft fleet. A particular problem is the formation of cracks, in particular fatigue cracks, and the growth thereof in different aircraft components. Cracks usually indicate imminent failure of the components affected, meaning that relevant components must be regularly checked, maintained and possibly even exchanged. Different approaches have already been developed in the past in order to reduce the growth of fatigue cracks in different aircraft components, such as for example the lower aircraft body, and thus to extend the service life of the components. Methods are particularly advantageous which can be used preventatively already before the formation of cracks, in order to reduce crack formation or, ideally, to prevent it. At the present time, so-called crack stoppers or crack delaying elements are usually used that are made for example from titanium or CFRP and are intended to reduce the growth of cracks. Since these methods usually involve the application of additional materials to the aircraft components, these methods generally lead to an increased weight of the respective component. Furthermore, welding of the joints between the aircraft skin and the crack stopper often requires great resources. All in all, the process thus requires very great resources and is expensive.
There is thus a great interest in developing cost-effective methods that are easy to carry out in order to reduce the speed of propagation of cracks, in particular fatigue cracks. Furthermore there is an interest in developing cost-effective methods that are easy to carry out, with which the substrate surface can be preventatively treated already before a crack arises, in order to already reduce the formation of a fatigue crack or, ideally, to completely prevent it. In this connection, residual stress-based methods represent a very promising approach. A particularly advantageous method for increasing the service life of the most varied components is the so-called laser heat treatment method. In this method, joints and residual stresses in a metal substrate are locally influenced close to the surface, whereby the speed of propagation of a crack is reduced. In this way, the service life of the metal substrate can be clearly increased without additional materials being welded or riveted. The original component, thus the metal substrate, is merely modified, whereby weight and costs can be saved.
Laser heat treatment methods have already been used to reduce the speed of propagation of cracks. U.S. Pat. No. 5,071,492 describes, for example, a method, wherein the substrate surface is heated parallel to the cracks arising, wherein a laser, inter alia, can also be used.
D. Schnubel et al. describe in “Materials Science and Technology A 546 (2012) 8-14” the use of a defocused laser to influence residual stresses in order to reduce the speed of propagation of fatigue cracks. The laser beam is thereby guided linearly over the substrate surface.
In the already known methods the laser beam is guided in points or lines over the substrate surface.
Within the scope of this invention it was unexpectedly shown that the effect of the residual stresses produced by means of laser heat treatment is significantly greater if the laser beam is not guided in the form of a line or a point, but instead in the form of a circle, an arc or a curve, over the material surface. It was found that the method according to the invention achieves significantly longer service lives of the treated materials at the same time as achieving low process and cost resources. Without wishing to be bound by a certain theory, it is assumed that this unexpected effect lies in the positive superimposition of the residual stress fields.